1. Field of Invention
The present invention relates to a semiconductor laser for the creation of light, including a semiconductor substrate, a laser layer arranged on the semiconductor substrate, a waveguiding layer arranged at least partially close-by the laser layer and a strip-shaped lattice structure. Furthermore the invention relates to a process for the fabrication of such a semiconductor laser.
2. Description of Prior Art
During the past years, laser diodes have been used in an increasing number of applications in different areas of technology. A major field of use is telecommunication technology, where such laser diodes are employed to transmit telephone calls and data. The light emitted from the laser diodes is transmitted via optical fibers to a receiver. Using optical transmission over fibers results in high transmission quality and a very high potential data transmission rate. While originally only one wavelength was used for fiber transmission, (that is, only the light of one laser diode with a single wavelength was transmitted), it has become more common in the last years to use multiple wavelengths for simultaneous transmission over optical fibers, so that many wavelengths contribute simultaneously to the transmission (wavelength multiplexing). With the simultaneous use of multiple wavelengths it is obviously possible to transmit higher data rates over a single optical fiber.
At the present state of technology, transmission using several wavelengths is usually achieved by merging the light emitted by several laser diodes with appropriate devices and then transmitting this light over a span of optical fiber. The single lasers emit light at different wavelengths. In order to achieve a high quality of data transmission and high data throughput it is necessary that the single laser diodes emit only light at the desired target wavelength. In practice, it cannot be avoided that a certain fraction of the light generated by the laser is also emitted at other wavelengths. The most important parameters with respect to the quality of the laser diodes include the so-called mono mode stability and side mode suppression ratio. The mono mode stability describes the deviation of the wavelength of emitted light under different operating conditions (temperature, applied voltage etc.). The side mode suppression ratio specifies the proportion of the light intensity at the strongest emitted wavelength in relation to the second strongest emitted wavelength. The larger the side mode suppression ratio, the less light is emitted in undesired frequency ranges. Other important factors include the change of the laser wavelength over the time of use.
Known laser diodes typically comprise active gain layers in which the light wave is amplified by stimulated emission. Especially in semiconductor lasers, this amplification is not strongly frequency selective, so that light is typically emitted over a broad frequency range. Therefore additional steps are necessary in order to achieve selectivity in frequency, that is to achieve light emission substantially at only one given wavelength. This wavelength or frequency is usually obtained by the use of periodic grating structures. The interference effects between the periodic grating structure and the lightwave causes wavelengths differing from the target wavelength to be strongly suppressed so that the emission is mainly amplified and emitted at the target wavelength.
At the present time it is generally assumed in the field that an exceedingly effective selection of the laser wavelength, and therefore a high side mode suppression, can only be achieved by using a very strong coupling between the lightwave and the periodic grating structure. This assumption is supported by a number of theoretical models and also experimental studies. The strength of the coupling is described by the so-called coupling coefficient κ which is usually chosen in the range between κ=100 cm−1 and κ=300 cm−1 or higher. For example, in the theoretical paper “Mode Selectivity of Distributed Bragg-Reflector Laser with Optical Loss in Corrugated Waveguide” by Masahiro Okuda et al, published in the Japan Journal of Applied Physics, Volume 14, 1975, No. 11, page 1859, an increased coupling coefficient resulted in an increased side mode suppression. The experimental work in the field is also based on the validity of this assumption. For example, in the article “Single and Tunable Dual-Wavelength Operation of an InGaAs-GaAs Ridge Waveguide Distributed Bragg Reflector Laser” by Roh et al in IEEE Transactions on Photonic Letter, Volume 12, No. 1, January 2000, page 16, the high side mode suppression ratio of the described laser diode is attributed to the relatively high value of the coupling coefficient κ.
Therefore, a need exists in the art for a semiconductor laser having improved performance in comparison with conventional semiconductor lasers including improved side mode suppression, single mode stability and cost-effective to fabricate and to operate.